A lower power driver will give you greater efficiency.
Try looking at the lower power Eminence 15" drivers
The Beta 15" can handle 100W in a bass cabinet.http://www.eminence-speaker.com/pdf/cab-beta-15a.pdf
2 of them in a cabinet would give you a few more dB.
Should be easy to optimise a design to make the best use of your 100W.
Another point to make is to use a steep highpass filter to conserve the power.